An electrophysiological study of putative neurotransmitters and histochemical localisation of biogenic amines in the central nervous system of the cockroach 'Periplanea Americana'

Abstract

A modification of the fluorescent histochemical glyoxylic acid (GA) method was used for localization of biogenic amines in wholemount ventral nerve cord and suboesophageal ganglia of the cockroach, Periplaneta americana. The distributions of fluorescent neurone somata within each ganglion and of nerve processes within the ganglia, interganglionic connectives and peripheral nerves were determined. An assessment was made of the effects of colchicine, nialamide plus DOPA and reserpine on the specific tissue fluorescence. The available evidence suggests that most of the specific fluorescence is due to dopamine. Infusion of hexamminecobaltic chloride was used to identify which fluorescent neurone pairs in the prothoracic and suboesophageal ganglia projected into the interganglionic connnectives and peripheral nerves. In order to determine the detailed morphology of certain of the fluorescent neurones GA was combined with intracellular injection of either Lucifer Yellow, hexamminecobaltic chloride, or Horseradish peroxidase. Technical difficulties, however, were encountered with the identification and filling of the neurones. A prothoracic neurone was identified which depolarized in response to pressure ejected and bath applied dopamine and noradrenaline. Both phentolamine and propranolol reversibly inhibited the dopamine response, although these drugs were also effective at antagonizing the cell's response to acetylcholine. Bath application of the drugs forskolin, IBMX and dibutyryl cyclic AMP provided evidence that in this case dopamine does not act through the second messenger, cyclic AMP. Additional electrophysiological experiments determined the effect of pressure ejected application of glutamate and GABA upon the electrical response of an identified metathoracic motoneurone. Picrotoxin reversibly inhibited the amino acid responses. Long term application of both picrotoxin and 4-acetamido-4' -isothiocyanostilbene-2,2'-disulphonic acid (SITS) produced changes in the membrane properties of the neurone such that the neurone somata became able to actively support action potentials.